The present invention relates to a bit error detection circuit for use in a digital communication system such as a TDMA (Time Division Multiplex Access) communication system, which checks the quality of a transmission path or line.
Generally, it is often required in digital communications lines to constantly monitor bit error rate (BER) on receiving lines. However, directly determining a BER from a received signal is in many cases impossible because the tranmsitted data is unknown. More accepted, therefore, is a method which predicts a BER of a received signal from information provided by comparing a main reproduced signal with a signal having a falsely deteriorated characteristic.
A prior art bit error detection circuit of the type described comprises, for example, first and second discriminator circuits adapted to discriminate an output of a phase demodulator with respect to first and second different reference voltages respectively, and a circuit for measuring a false BER by comparing an output of the first discriminator circuit with an output of the second discriminator circuit. A problem encountered with such a circuit construction is that in the TDMA communication of a burst PSK (phase Shift Keying) modulated carrier wave for example, the burst carrier wave level and, therefore, the predicted BER differs from one station to another.
Another prior art bit error detection circuit comprises first and second discriminator circuits adapted to discriminate the output of a phase demodulator at a time corresponding to the center .theta..sub.0 of data (bit) and a time deviated by a phase .theta..sub.1 from the center .theta..sub.0 respectively, and a circuit for measuring a pseudo BER by comparing the output of the second discriminator circuit with the output of the first discriminator circuit. This is not acceptable in the case of a transmission line which includes a high power amplifier or like non-linear element because, as will be described in detail, the operating point of a non-linear element changes depending upon the operation mode, i.e. linear mode or non-linear mode. Such is presumably attributable to a change in the amount of variation in intersymbol interference from the phase point .theta..sub.0 to the phase point .theta..sub.0.